The manufacturers' organisation CCFA said the plunge was less brutal if two extra working days in September 2011 were factored in, lowering the figure to 10%.
Sales of new cars showed a drop of 13% for the first nine months of the year, it said.
The sales of French carmakers suffered more than those of their European counterparts in the three quarters of this year at an average of 18.9% against 7.3% for the others.
The CCFA also revised its forecast for annual sales, predicting a decline of 12% against 8% to 10% forecast earlier.
But CCFA president Patrick Blain warned the new figure would only hold if the climate "does not deteriorate in the coming months."
Renault's sales in September plunged by a third in September to 31,929 vehicles.
PSA Peugeot Citroen, the biggest French car maker and second-biggest in Europe after the German Volkswagen group, fared better with a 5% decline.
The key French sector has been weak for several months following the end of a cash-for-clunkers government bonus to support car sales at the height of the eurozone financial crisis.
PSA Peugeot Citroen last week forecast zero growth next year in the European market, which accounts for the bulk of its sales, while Renault has underscored the need for improved competitivity to remain afloat.